Cranked it down further, to Kindergarten, just to get this done, catch up and actually enjoy the story without the pain of repetition. And it worked wonders. M1 was always so much harder on TC than M2 or M? and I guess with this one they just kept up that tradition.
The punching assimilated guy was an interesting concept, especially as it was unexpected and not that easy to notice - playing on M1 under Classic, I'm not getting injury screen flashes :(.
As for the contrivedness of the path - certainly not quite on the same level as Habe Quiddam or similar, but just as annoying with the lava playing a prominent role.